Amorphization Dose Research Articles

Structural characterization of high-dose C ++N + ion-implanted (1 1 1) Si

(1 1 1) silicon wafers were implanted with high ion doses of C +,N + and C ++N +, from 1×10 16 to 6.7×10 17 cm −2 , well over the amorphization dose. For the largest dose, the mean impurity concentration in the implanted region (of about 1000 Å) at 30 keV is expected to be similar to the Si atom density. Silicon nitride and silicon carbide formation is detected in the as-implanted samples. After annealing at 1200 °C a trend to segregation of Si 3N 4 and SiC in separated layers is observed, and a polycrystalline Si overlayer is formed.

Temperature effects on the radiation stability and ion exchange capacity of smectites

Radiation stability of montmorillonite, nontronite, and saponite as a function of temperature has been investigated by in situ transmission electron microscopy (TEM) with 200 keV electrons. All three phases underwent a crystalline-to-amorphous transformation when exposed to electron beam irradiation over the temperature range of 25–750°C. At room temperature, the amorphization doses of montmorillonite, nontronite and saponite are 1.3×10 24, 6.2×10 24 and 1.7×10 24 e −/ m 2 , which correspond to energy depositions of 3.0×10 10, 3.4×10 11 and 6.0×10 10 Gy, respectively. In the low-temperature region (25–400°C), the amorphization doses of all three types of smectite increase with the increase of temperature. Further increase of temperature to above 400°C leads to a decrease in the amorphization dose. The temperature effect on the ion exchange capacity for Cs has been studied using thermally treated smectites at temperatures between 25°C and 900°C. No change in ion exchange capacity was found at temperatures below 400°C, for which dehydration is the dominant process during the heat treatment. Above this temperature, dehydroxylation, amorphization and phase transformation occur depending on temperature. The ion exchange capacity decreases with the increase in temperature, as a result of the loss of exchangeable cations in the crystalline structure.

Radiation damage and nanocrystal formation in uranium–niobium titanates

Two uranium–niobium titanates, U 2.25Nb 1.90Ti 0.32O 9.8 and Nb 2.75U 1.20Ti 0.36O 10, formed during the synthesis of brannnerite (UTi 2O 6), a minor phase in titanate-based ceramics investigated for plutonium immobilization. These uranium titanates were subjected to 800 keV Kr 2+ irradiation from 30 to 973 K. The critical amorphization dose of the U-rich and Nb-rich titanates at room temperature were 4.72×10 17 and 5×10 17 ions/ m 2 , respectively. At elevated temperature, the critical amorphization dose increases due to dynamic thermal annealing. The critical amorphization temperature for both Nb-rich and U-rich titanates is ∼933 K under a 800 keV Kr 2+ irradiation. Above the critical amorphization temperature, nanocrystals with an average size of ∼15 nm were observed. The formation of nanocrystals is due to epitaxial recrystallization. At higher temperatures, an ion irradiation-induced nucleation-growth mechanism also contributes to the formation of nanocrystals.

Damage accumulation and recovery in gold-ion-irradiated barium titanate

Single-crystal barium titanate (BaTiO 3) wafers were irradiated 60° off the surface normal at 170 and 300 K using 1.0 MeV Au 2+ ions over a fluence range from 0.03 to 0.19 ions/nm 2 . Disorder on both the Ba and Ti sublattices has been studied in situ using Rutherford backscattering spectrometry along the 〈1 1 0〉 axial direction. At these irradiation temperatures, the temperature dependence of disordering is small. The dose for amorphization under these conditions is on the order of 0.5 dpa, which is 50% of that required to amorphize SrTiO 3 under similar conditions. At low damage levels, recovery of disorder is observed at room temperature, suggesting at least one lower temperature recovery stage. For more highly damaged states, two distinct recovery stages have been identified between 420 and 570 K and between 720 and 870 K. The recovery stage between 420 and 570 K is associated with the critical temperature for full amorphization (∼550 K) in BaTiO 3. The higher temperature recovery stage is most likely associated with epitaxial recrystallization.

Ion irradiation-induced amorphization of two GeO 2 polymorphs

The amorphization of two GeO 2 polymorphs, the hexagonal ( P3 2 21) and tetragonal forms ( P4 2 /mnm), were studied using 1.5 MeV Xe +. The polymorphs showed significant differences in their susceptibility to amorphization by irradiation. Hexagonal GeO 2 had a lower amorphization dose (1.7×10 13 ions/cm 2 at room temperature), higher critical temperature (876 K) and higher activation energy for dynamic annealing (0.2 eV) than tetragonal GeO 2 which had a higher amorphization dose (1×10 15 ions/cm 2 at room temperature), lower critical temperature (820 K) and lower activation energy (0.02 eV). Because of their identical composition, the difference in susceptibility to amorphization between the two GeO 2 polymorphs is attributed to the connectivity of polyhedra. The glass-forming ability of hexagonal GeO 2 is much greater than tetragonal GeO 2 because of the three dimensional network formed by corner sharing (GeO 4) tetrahedra. The radiation effects of two GeO 2 polymorphs are also compared to the results of irradiation study of two related materials, rutile (TiO 2) and quartz (SiO 2). Both chemical composition and structure contribute significantly to the susceptibility to irradiation-induced amorphization in these phases.

Compositional dependence of defect mobility and damage buildup in Al xGa 1− xAs

Defect transformations at low temperatures in ion implanted Al x Ga 1− x As (0⩽ x⩽1) ternary compounds were studied. Experiments consisted of ion implantation with 150 keV N or 200 keV Ar ions with different doses at temperatures between 18 and 77 K, and in situ RBS/channeling measurements at selected temperatures. An important recovery stage attributed to the defect mobility in the Ga(Al) sublattice was revealed near 280 K. For x>0.5 this stage was largely suppressed. Instead, a continuous damage recovery at low temperatures was observed. It was noticed that defect recombination can also be produced upon prolonged storage at the implantation temperature. For AlAs ( x=1) the 280 K stage disappeared completely and only a small defect recovery at low temperatures was noticed. Upon N- or Ar-ion bombardment, after an incubation period, a sharp crystalline-to-amorphous transition appeared. The amorphization dose increases with increasing x and is a factor of 10 higher for x=0.96 than that for x=0. A further increase of the dose by a factor of 15 was required to amorphize AlAs ( x=1).

Effect of implant temperature on secondary defects created by MeV Sn implantation in silicon

Secondary defects induced by ion implantation in silicon after annealing have been previously shown to vary with the implantation and annealing conditions. However, in the low dose implants, well below the amorphization dose, the defects have been predominantly characterized to be interstitial in nature. In this article, we study the effect of implant temperature on secondary defects created by 1 MeV Sn implantation to a dose of 3×1013 cm−2 after subsequent annealing. We report a variation in the defect microstructure with implant temperature showing preferential formation of small interstitial loops for −191 °C and only rod-like defects for similar implants carried out at 300 °C. We conclude that these microstructures are a result of the dense cascades created by heavy Sn ions, creating local amorphous pockets in the implant damage region at the lowest implant temperatures. The variation of the microstructure with implant temperature is interpreted in terms of the effect of dynamic annealing over the defects formed in silicon.

Irradiation-induced amorphization ofCd2Nb2O7pyrochlore

Several investigations have recently been undertaken in order to achieve a more complete understanding of the radiation-damage mechanisms in ${\mathrm{A}}_{2}{\mathrm{B}}_{2}{\mathrm{O}}_{7}$ pyrochlore-structure compounds. The present work represents the first systematic study of the irradiation-induced amorphization of a pyrochlore with A- and B-site cation valences of +2 and +5, respectively. Relatively large single crystals of ${\mathrm{Cd}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ were grown for these experiments. In situ ion-irradiation experiments were carried out in a transmission electron microscope in conjunction with ex situ Rutherford backscattering measurements of ion-irradiated ${\mathrm{Cd}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ single crystals. ${\mathrm{Cd}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ can be amorphized in situ by Ne or Xe ions at temperatures up to 480 and 620 K, respectively. At room temperature, the amorphization fluence was 36 times higher for 280 keV ${\mathrm{Ne}}^{+}$ than for 1200 keV ${\mathrm{Xe}}^{2+},$ corresponding to a displacement dose that was higher by a factor of 3. Disordering of Cd and Nb over the available cation sites occurs at intermediate ion doses prior to amorphization. The temperature dependence of the amorphization dose is modeled, and the results are compared to those of a previous model. The bulk-sample Rutherford backscattering spectroscopy (RBS) results were generally consistent with the in situ TEM measurements. Effects of crystallographic orientation and ion charge state had relatively little effect on the damage accumulation in bulk crystals. The RBS data are consistent with a defect-accumulation, cascade-overlap model of amorphization of ${\mathrm{Cd}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7},$ as are the in situ TEM observations.

Heavy-ion irradiation effects on structures and acid dissolution of pyrochlores

The temperature dependence of the critical dose for amorphization, using 0.6 MeV Bi + ions, for A 2Ti 2O 7 pyrochlores, in which A=Y, Sm, Gd and Lu, exhibits no significant effect of A-site ion mass or size. The room temperature dose for amorphization was found to be ∼0.18 dpa in each case. After irradiation with 2 MeV Au 2+ ions glancing-incidence X-ray diffraction (XRD) revealed that each pyrochlore underwent an irradiation-induced structural transformation to fluorite in conjunction with amorphization. The effect of amorphization on the dissolution rates of fully dense pyrochlores, at 90°C and pH 2 (nitric acid) varied from a factor of 10 to 15 increase for Gd 2Ti 2O 7 to none for Y 2Ti 2O 7. Significant differences were observed in the A-site dissolution rates from the crystalline pyrochlores, indicating differences in the manner in which the A-site cations are incorporated into the pyrochlore structure. These indications were supported by Raman spectroscopy.

Molecular dynamics simulation of irradiation-induced amorphization of cubic silicon carbide

It has long been observed that a crystalline-to-amorphous (c-a) transition occurs in silicon carbide (SiC) irradiated at low temperature. However, the microscopic mechanisms leading to the transition are not well understood. We report in this paper a molecular dynamics (MD) simulation of low-energy (100 eV) recoil accumulation at cryogenic temperature (20 K), up to ≈1 dpa, in which the irradiated computational sample becomes amorphous and is subsequently annealed at high temperature (2320 K). The simulation suggests that, at least for low-mass impinging particles, provided that no direct impact amorphization (DIA) takes place, the driving force for the c-a transition in this material is the accumulation of Frenkel pairs up to a critical concentration ( ≈1.9×10 22 cm −3 ). The role of antisites in the process is negligible. In fact, antisite formation during the annealing could be the bottleneck for complete recovery. A simple and intuitive analytical model based on the concepts of recombination barriers and interstitial migration is also proposed, to describe the temperature dependence of the critical dose for amorphization.

Doping characteristics of BF/sub 2//sup +/ implants in >100< and >111< silicon

Significant differences were noticed between doping characteristics of BF/sub 2//sup +/ implanted [100] and [111] silicon. High dose BF/sub 2//sup +/ implants always produced higher sheet resistance (R/sub s/) in [111] silicon. Compared to B/sup +/, the BF/sub 2//sup +/ implants exceeding the amorphization dose always resulted in higher R/sub s/ regardless of silicon orientation, Behavior was attributed to fluorine gettered at implant-induced defect sites. Sheet resistance of an F/sup +/+B/sup +/ implanted layer was found to increase linearly with F/sup +/ dose. Residual fluorine content decreased with temperature, especially for [111] orientation, and coincided with a decrease in R/sub s/. Effects of fluorine and crystal damage were separated experimentally and correlated to R/sub s/ of implanted layers. SIMS analysis revealed that fluorine segregated at two peaks in [111] silicon and four peaks in [100] silicon, Experimental results suggest that gettered fluorine and boron-fluorine bonds formed at heavily damaged crystal sites prevent boron from its full electrical activation in BF/sub 2//sup +/ implanted samples.

Irradiation-induced amorphization: Effects of temperature, ion mass, cascade size, and dose rate

An empirical model based on cascade ``quenching'' and epitaxial recrystallization has been developed to describe the accumulation of the amorphous fraction during ion beam irradiation experiments. The model is based on the assumption that the amorphous fraction that remains after the formation of a cascade is related to a crystallization efficiency parameter A. For low values of A, as would be expected at low temperatures, for heavy-ion irradiations, or for materials that are good glass formers, the accumulation of the amorphous fraction as a function of dose is an exponential function. For high values of A, as would be expected at elevated temperatures, for light-ion irradiations, or for materials that are poor glass formers, the accumulation of the amorphous fraction as a function of dose is a sigmoidal function. Amorphization dose varies as a function of temperature and is reflected by the temperature-dependent crystallization efficiency. The effects of ion mass and energy on critical amorphization dose and temperature are discussed in terms of the cascade size. The dose-rate effect on the critical temperature of amorphization is derived considering the thermal annealing of the damaged material.

Compositional variation of microstructure in ion-implanted AlxGa1−xAs

The ion damage produced in alloys of AlxGa1−xAs (x = 0.6, 0.7, 0.8, and 0.85) by implantation at 77 K with Kr ions (500, 700, and 1500 keV) was studied by using Rutherford backscattering channeling and transmission electron microscopy. In addition, the accumulation of ion damage at 50 K was studied by performing the ion implantations in situ in the transmission electron microscope. In Al0.8Ga0.2As, damage accumulation at 77 K was independent of dose rate, indicating that dynamic annealing is not occurring at 77 K. The in situ studies demonstrated that planar defects are produced on warm-up from 50 K to room temperature, indicating that they are not the nucleation site for amorphization. The lower energy implantations revealed that amorphization initiated within the AlxGa1−xAs layer, showing that heterointerfaces are not required for amorphization. These results, along with the similarity of the room-temperature microstructures in the different alloys, imply that the amorphization mechanism is independent of Al content. It is proposed that the observed dependence of the amorphization dose on Al content is related to an increase in the number of cascade overlaps needed to initiate and to produce a continuous amorphous layer. A mechanism explaining the microstructural changes with composition, based on the thermal and physical properties of the alloy and on the distribution of energetic cascade events, is presented.

Nano-scale glass formation in pyrochlore by heavy ion irradiation

Irradiation-induced amorphization in a pyrochlore (Na,Ca) 2Nb 2O 6(F,OH,O) and a microlite (Na,Ca) 2 Ta 2O 6(F,OH,O) have been studied over a temperature range of 300–873 K by in situ transmission electron microscope (TEM) using 1 MeV Kr + and 400 keV Xe + with a dose rate of 8.5×10 11 ions/cm 2/ s. Isolated nanometer-sized domains of glass were observed by high-resolution electron microscopy in the Xe +-irradiated samples. The density of these domains has a ‘one-to-one’ correlation to the ion density, consistent with the amorphous domains having formed directly within the displacement cascade. The effect of temperature on the amorphization dose was modeled based on a cascade quenching mechanism. A two-stage function was observed in the temperature dependence of amorphization dose for both the pyrochlore and microlite.

HREM and Image Simulation of Short-Range-Order Domains in Ion Irradiated Zirconolite

Abstract Zirconolite (CaZrTi2O7) is an important phase proposed for immobilization of plutonium. Radiation effects in zirconolite were studied by 1 MeV Kr+ and 1.5 MeV Xe+ irradiation at various temperatures. Zirconolite became amorphous at temperatures below a critical temperature, Tc. The critical temperature was found to be a function of ion species: Tc - 654 K for 1 MeV Kr+ and 710 K for 1.5 MeV Xe+. The temperature dependence of amorphization dose is shown in FIG. 1. Above Tc, the specimen remained crystalline after prolonged irradiation (up to 3.6×l015 ions/cm2). However, the high-temperature irradiated zirconolite was transformed into the fluorite structure (as shown by the strong diffraction maxima in FIG. 2). In addition to the maxima from the fluorite structure, strong diffuse maxima were observed surrounding the Bragg position of pyrochlore superlattice (FIG. 2).

Ion irradiation of rare-earth- and yttrium-titanate-pyrochlores

Pyrochlore, A 1–2B 2O 6(O,OH,F) 0–1, is an actinide-bearing phase in Synroc, a polyphase ceramic proposed for the immobilization of high level nuclear waste. Structural damage due to alpha-decay events can significantly affect the chemical and physical stability of the nuclear waste form. Pyrochlore can effectively incorporate a variety of actinides into its structure. Four titanate pyrochlores were synthesized with compositions of Gd 2Ti 2O 7, Sm 2Ti 2O 7, Eu 2Ti 2O 7 and Y 2Ti 2O 2. These samples were irradiated with 1 MeV Kr + in order to simulate alpha-decay damage and were observed by in situ electron microscopy. Irradiations were conducted from 25 K to 1023 K. At room temperature, Gd-, Sm- and Eu-pyrochlores amorphized at a dose of ∼2×10 14 ions/cm 2 (∼0.5 dpa) and Y-pyrochlore amorphized at 4×10 14 ions/cm 2 (∼0.8 dpa). The amorphization dose became higher at elevated temperatures with different rates of increase for each composition. The critical temperatures for amorphization are ∼1100 K for Gd-, Sm-, Eu-pyrochlore and ∼780 K for Y-pyrochlore. The rare-earth-pyrochlores are more susceptible to amorphization and have higher critical temperatures than Y-pyrochlore. The difference in amorphization dose and critical temperature is attributed to the different cascade sizes caused by the different cation masses of the target. Based on a model of cascade quenching, the larger cascade is related to a lower amorphization dose and higher critical temperature. The irradiated materials were studied by electron diffraction and high-resolution electron microscopy. All the pyrochlores transformed to a fluorite substructure prior to the completion of amorphization of the observed regions. This transformation was caused by the disordering between cations and between oxygen and oxygen vacancies. The concurrence of cation disordering with amorphization suggests the partial recrystallization of the displacement cascades. Isolated cascade damage regions were observed by high-resolution electron microscopy, and the cation disordering was associated with the damaged regions.

Structural and electrical properties of CVD diamond films doped by N + implantation

Chemical vapor deposited (CVD) diamond films (DF) were prepared on P-type <100> oriented Si substrates. The films were implanted with 140 keV N + to doses of 5×10 14∼1×10 16cm −2, respectively. Under the implantation, the DF becomes more and more disordered with each increasing dose. Scanning electronic microscopy (SEM) and Raman measurements reflect the amorphization process. Combining the electrical resistance measurements, it is clear that polycrystalline DF has a higher threshold dose of amorphization than that for bulk ones. The damages induced by implantation can be partially removed by annealing when the dose is below the critical value. Larger doses of implantation produce amorphous carbon which can transform into micro-polycrystalline graphite during annealing. Thus, the conductivity is increased dramatically. However, for smaller dose N + implanted samples, while annealing was performed, the samples become comparable conductive for the activation of N dopants.

Ion irradiation-induced amorphization of six zirconolite compositions

Zirconolite is an important phase in Synroc, a polyphase titanate ceramic, designed for the immobilization of high level waste from nuclear fuel reprocessing. Zirconolite is a principal host phase for actinides in Synroc. We have studied radiation effects of six zirconolite compositions: CaZrTi2O7, Ca0.8Ce0.2ZrTi1.8Al0.2O7, Ca0.85Ce0.5Zr0.65Ti2O7, Ca0.5Nd0.5ZrTi1.5Al0.5O7, CaZrNb0.85Fe0.85Ti0.3O7 and CaZrNbFeO7. The samples have been irradiated by 1.0 MeV Kr+in a temperature range from 25 to 973 K and observed by in situ transmission electron microscopy. The radiation-induced crystalline-to-amorphous transformation was studied by electron diffraction and high-resolution electron microscopy. Concurrent with amorphization, monoclinic zirconolite has been found to transform to a fluorite sublattice through cation disordering during irradiation. All the zirconolites amorphized after a similar dose (∼2×1015−3.9×1015ions/cm2) at room temperature. The amorphization dose increased at elevated temperatures with varying rates of increase for each phase. The critical temperatures for amorphization were 550 K for CaZrNbFeO7, 590 K for CaZrNb0.85Fe0.85Ti0.3O7, 640 K for CaZrTi2O7, 900 K for Ca0.8Ce0.2ZrTi1.8Al0.2O7, 1000 K for Ca0.85Ce0.5Zr0.65Ti2O7, and 1020 K for and Ca0.5Nd0.5ZrTi1.5Al0.5O7. The trend of the critical temperatures indicates that decreasing calcium content increases susceptibility to amorphization. The role of calcium in the radiation-induced amorphous structure is considered to be that of a network-modifier of the aperiodic structure formed by the polyhedra of high-valence cations.

Special features of spatial redistribution of selenium atoms implanted in silicon

The spatial distribution of selenium atoms implanted in silicon was studied by secondary-ion mass spectrometry after annealing in the temperature range of 600–1200°C. For implantation doses exceeding the amorphization dose for silicon, formation of a peak of selenium concentration was detected beyond the mean projected range of selenium ions. The spatial position of the peak correlates well with the spatial position of the plane in which the calculated value of the specific energy losses of selenium ions in elastic collisions (according to the TRIM code) corresponds to the critical value for amorphization of silicon. Accumulation of impurities at the peak occurs at temperatures of 700°C and higher after recrystallization of the amorphized layer. Redistribution of selenium atoms to deeper layers of the sample due to diffusion is controlled by the temperature dependence of the solubility of selenium in silicon.

Electron and ion irradiation of zeolites

Three zeolites (analcime, natrolite, and zeolite-Y) were irradiated with 200 and 400 keV electrons. All zeolites amorphized at a relatively low electron fluence. The electron fluences for amorphization by the 200 keV electron irradiation at room temperature were: 7.0×10 19 e −/ cm 2 (analcime), 1.8×10 20 e −/ cm 2 (natrolite), and 3.4×10 20 e −/ cm 2 (zeolite-Y). These doses are equivalent to an energy deposition between 2.6×10 10 and 1.27×10 11 Gy. An inverse temperature dependence of amorphization dose was observed for all three zeolites, i.e., amorphization dose decreased with increasing temperature. Analcime was also irradiated with 1.5 MeV Kr + from 300 to 973 K. The amorphization dose by the ion irradiation was constant, ∼1×10 14 ions / cm 2 , which is ∼0.1 dpa of collisional damage and ∼6.25 × 10 8 Gy of ionizing energy deposition.